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Q&A How to write a macro that discards the const qualifier, for any type?

Ignoring the numerous forms of undefined behavior that casting away const might invoke, the blunt but simple and standard solution is just to cast to (void*). char* foo (const char* str) { r...

posted 2y ago by Lundin‭  ·  edited 2y ago by Lundin‭

Answer
#2: Post edited by user avatar Lundin‭ · 2022-06-15T19:55:13Z (almost 2 years ago)
  • Ignoring the numerous forms of undefined behavior that casting away `const` might invoke, the blunt but simple and standard solution is just to cast to `(void*)`.
  • ```c
  • char* foo(const struct t *r)
  • {
  • return (void*)r->s;
  • }
  • ```
  • This is far more portable than gcc extensions like `typeof`.
  • (C23 might introduce various type-related features similar to `_Generic` so there might be more elegant ways coming soon.)
  • The only application for these kind of dirty casts is pretty much when dealing with broken API functions that take const-correct parameters but return a non-const pointer, like for example:
  • char *strstr (const char *s1, const char *s2);
  • This function is broken by design - always was. Correct design would be to return an index instead of a pointer. But if you are stuck with a broken API such as this one and have to implement it, you have to take shortcuts like casting away const.
  • Ignoring the numerous forms of undefined behavior that casting away `const` might invoke, the blunt but simple and standard solution is just to cast to `(void*)`.
  • ```c
  • char* foo (const char* str)
  • {
  • return (void*)str;
  • }
  • ```
  • This is far more portable than gcc extensions like `typeof`.
  • (C23 might introduce various type-related features similar to `_Generic` so there might be more elegant ways coming soon.)
  • The only application for these kind of dirty casts is pretty much when dealing with broken API functions that take const-correct parameters but return a non-const pointer, like for example:
  • char *strstr (const char *s1, const char *s2);
  • This function is broken by design - always was. Correct design would be to return an index instead of a pointer. But if you are stuck with a broken API such as this one and have to implement it, you have to take shortcuts like casting away const.
#1: Initial revision by user avatar Lundin‭ · 2022-06-15T19:53:47Z (almost 2 years ago)
Ignoring the numerous forms of undefined behavior that casting away `const` might invoke, the blunt but simple and standard solution is just to cast to `(void*)`.

```c
char* foo(const struct t *r)
{
    return (void*)r->s;
}
```

This is far more portable than gcc extensions like `typeof`.



(C23 might introduce various type-related features similar to `_Generic` so there might be more elegant ways coming soon.)

The only application for these kind of dirty casts is pretty much when dealing with broken API functions that take const-correct parameters but return a non-const pointer, like for example:

    char *strstr (const char *s1, const char *s2);

This function is broken by design - always was. Correct design would be to return an index instead of a pointer. But if you are stuck with a broken API such as this one and have to implement it, you have to take shortcuts like casting away const.